The term "engine oil" as used hereinbefore and hereinafter means a lubricating oil that may be useful in an engine oil, and by way of example, includes an automotive oil or diesel engine oil, including both formulated and virgin oils.
Among the materials that impart detergency to lubricating oils to keep internal engine parts clean and reduce sludge formation in the oil are overbased detergents, particularly calcium sulfonates. These sulfonates are known to be useful as additives for lubricating oils, particularly as a crankcase engine oil for internal combustion engines.
It is known that equivalent detergency characteristics can be obtained with a lower concentration of additive in a lubricating oil--the higher the alkaline reserve of an additive: the larger the quantity of acidic combustion products accumulated in the oil to which the additive is added that can be neutralized by the additive. The measurement of alkaline reserve is reported as total base number (TBN) which is the number of milligrams of potassium hydroxide equivalent to the amount of acid required to neutralize the alkaline constituents present in one gram of sample. An additive having a total base number higher than can be obtained from calcium petroleum sulfonate alone is commonly said to be "over-based" or, alternatively, is said to be "superbasic".
Overbased calcium sulfonates are generally produced by carbonating a mixture of hydrocarbons, sulfonic acid, calcium oxide or calcium hydroxide and promoters such as methanol and water. In carbonation, the calcium oxide or hydroxide reacts with the gaseous carbon dioxide to form calcium carbonate. The sulfonic acid is neutralized with an excess of CaO or Ca(OH).sub.2 to form the sulfonate. The prior art known processes for overbasing calcium sulfonates produce high alkaline reserves of TBN of 300 to 400 mg KOH/gm or higher, which enables the formulator to use lower amounts of additive while maintaining equivalent detergency to protect the engine adequately under conditions of high acid formation in the combustion process.
The calcium carbonate component of the overbased calcium sulfonate forms the core of a calcium sulfonate micellar structure. The calcium carbonate is either in the amorphous and/or one or more of its crystalline forms particularly, calcite.
Papke, et al., U.S. Pat. No. 4,995,993, recognized that large micellar crystalline calcium carbonate structures caused haze, and overbased sulfonate products containing crystalline calcium carbonates are always undesirable and therefore crystallization was to be avoided at all costs. See, Papke, et al. at col. 4, lines 39-42. Papke, et al. consequently directed one to a product that contains an amorphous calcium carbonate core micellar structure of 100 to 150 Angstroms in size for 400 TBN products, whereas crystalline-core calcium sulfonates were found to always have large micellar sizes of 400 to 600 Angstroms. See, Papke, et al. at col. 4, line 53 to col. 5, line 5. Papke, et al. also found that even where small crystalline-core calcium sulfonate micelles were first formed, agglomeration readily effected undesired large micelles. See, Papke, et al., e.g. at col. 5, lines 4-14. The prior art could not tolerate more than about 1% by weight of calcite in a lubricating oil.
It was also recognized in the art, as disclosed in "Colloidal anti-wear additives 2. Tribological behavior of colloidal additives in mild wear regime," J. L. Mansot, et al., Colloids and Surfaces A: Physico Chemical and Engineering Aspects, 75 (1993), pp. 25-31, that overbased micelles composed of an amorphous calcium carbonate core surrounded by calcium didodecylbenzene sulfonate molecules strongly bonded to the core, when in a 2% by weight dispersion in dodecane, and subjected to metallic friction surfaces, the calcium carbonate forms a polycrystalline film adherent to the metallic friction surfaces, which resultantly provides anti-wear protection. Mansot, et al. thereby directed one to providing an overbased calcium sulfonate with an amorphous micellar structure which would then, under a mild wear regime, undergo transformation to microcrystalline agglomerates through an amorphous intergranular phase. Mansot, et al., in this manner, further confirmed the direction of the prior art to providing amorphous calcium carbonate micellar dispersion overbased calcium sulfonate detergents.
Prior art crystalline overbased calcium sulfonates were hazy and not oil soluble. Such prior art crystalline overbased calcium sulfonates are disclosed in U.S. Pat. No. 4,560,489 to Muir, et al.; U.S. Pat. No. 3,242,079 to McMillen; and U.S. Pat. No. 3,376,222 to McMillen. These products were used as additives for greases, paints (for rheology control) and in extreme pressure (EP) metal working formulations. The prior art calcite overbased calcium sulfonates, such as disclosed in Muir, et al. were hazy and had particle sizes ranging from 50 Angstroms up to 5,000 Angstroms, with minimum viscosities of 1 million to 10 million cps at 25.degree. C. Typically, products containing such calcite overbased calcium sulfonates were rheology modified greases.
In the art directed to extreme pressure (EP) lubricants, particularly including metal working fluids and greases, where haze free aesthetics was not a commercial consideration, it was known to provide calcite-core overbased calcium sulfonate detergents for improved anti-wear properties. That is, it was recognized that the calcite contributed to improved anti-wear in such lubricants. These lubricants however were hazy, and for the foregoing and following reasons were precluded from use in automotive crankcase or like engine oils.
The lubricating oil art, particularly as directed to automotive crankcase and other engine oils, mandated a clear or substantially haze free product for requisite consumer aesthetics and acceptance. This need precluded the use of prior art detergents with haze producing crystalline calcium carbonate. The art recognized haze test was the Hazitron test, as further discussed hereinafter. Hazitron test values of generally less than 30, and more usually less than about 15 to 10, were considered commercially substantially haze free and acceptable. Additionally, engine oils desirably had reduced turbidity, as measured by a turbidimeter, of below 100, preferably below 40 to 60, and most preferably below 25.
The automotive engine or motor oil art solution to providing requisite anti-wear was the addition of a zinc dithiophosphate (ZDP) to the motor oil. While ZDP provided anti-wear improvement, it was an otherwise undesirable solution in that; (1) ZDP attacked the catalyst in a catalytic converter which in turn resulted in pollution emissions, and (2) ZDP effectiveness was reduced by the co-presence of the overbased hydrocarbyl sulfonates (as discussed in e.g., Yamaguchi, et al., U.S. Pat. No. 4,668,409). In order for motor oils to pass the mandated engine tests, the motor oils required at least about 0.1% by weight phosphorous (P) ZDP.
The art desired a lubricating oil detergent with inherent improved anti-wear properties, which also necessarily had commercially acceptable levels of minimal haze, or were essentially haze free, and with acceptable minimal levels of turbidity. The automotive oil art particularly desired an as aforesaid improved anti-wear detergent for use as a crankcase engine oil.